Remove temporary default case for PointerSignalKind (#128900)

This PR cleans up some prep that was added as part of
https://github.com/flutter/flutter/pull/120731 which was done to unblock
https://github.com/flutter/engine/pull/39637
Since the work done in that PR was reverted, this should be removed.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

packages/flutter/lib/src/gestures/converter.dart

@@ -304,11 +304,6 @@ abstract final class PointerEventConverter {
                 scale: datum.scale,
               );
             case ui.PointerSignalKind.unknown:
-            default: // ignore: no_default_cases, to allow adding a new [PointerSignalKind] - PointerStylusAuxiliaryAction
-            // TODO(louisehsu): remove after landing engine PR https://github.com/flutter/engine/pull/39637
-              // This branch should already have 'unknown' filtered out, but
-              // we don't want to return anything or miss if someone adds a new
-              // enumeration to PointerSignalKind.
               throw StateError('Unreachable');
           }
         }).whereType<PointerEvent>();